It is known to produce light-emitting devices whose color is obtained and controlled by mixing primary hues. The combination of three light-emitting diodes (LEDs) each emitting at a specific wavelength corresponding to blue, to green and to red makes it possible to obtain any color provided that the intensity ratio between each LED is well controlled. This property is particularly beneficial for fabricating screens or lamps with variable hues and intensities (“intelligent lighting”). For applications of this type, it is possible to assemble three distinct LEDs, which can if necessary be fabricated from distinct semi-conducting materials. The commonest configuration is for example to use two LEDs based on nitrides (AlInGaN) for blue and green, and a phosphide LED (AlGaInP) for red. However, the use of three different LEDs has a non-negligible cost overhead with respect to a single LED. Moreover, combining three distinct LEDs for the production of each pixel poses miniaturization problems which become insoluble for applications of screen type with a low pixel size.
These miniaturization problems can be solved by using organic LEDs (“OLEDs”), which make it possible to readily associate three or four emitters of different colors in each pixel of an active-matrix screen. However, the luminance of OLEDs is too low for certain applications, such as lighting or “near the eye” projection.
It has also been proposed to produce a stack of inorganic semi-conducting layers comprising three or more LEDs, emitting at different wavelengths, that can be controlled independently of one another. In this manner, a single stack constitutes a complete pixel, in which the various sub-pixels are superposed instead of being arranged alongside one another. See in this regard U.S. Pat. No. 8,058,663. Such a structure is difficult to produce, and above all to hybridize, since it is necessary to independently interconnect several active layers situated at different depths of the stack. Moreover, the structures described by document U.S. Pat. No. 8,058,663 comprise either a plurality of tunnel junctions intended to be connected in series, or contacts taken on “stairways” etched into semi-conducting layers exhibiting a doping of type p; in both cases, relatively significant electrical resistances are obtained, inducing high losses. Furthermore, the depths to be etched are relatively significant, thus putting limits on the miniaturization achievable.
Documents US 2011/233575 and EP 2 187 442 use luminophores to perform wavelength conversions, thus making it possible to obtain various colors from one and the same electroluminescent structure. However, the thickness of the luminophore layers must be large enough (of the order of some hundred μm) to make provision to absorb the light emitted by the active layer injected electrically. Such a thickness is not compatible with a high degree of miniaturization of the pixels (lateral dimensions of the order of some ten μm) because of cross absorption of light between the various pixels.
Documents WO 2010/123814 and US 2011/0256648 teach the use of semi-conducting wavelength converters with the same aim. The structures described in these documents cannot be considered to be monolithic; their fabrication requires gluing steps which are complex to carry out.